I was going to make this thread myself, but seeing as it's already here, I might as well post my two cents, and check on some of these iconic moments myself.

My opinion of most iconic moments in anime.:

Lalah's death (Mobile Suit Gundam)
A defining moment in anime just as Gwen Stacy's was a defining moment in American comics. The death of Lalah was the high point of all the tragedies of war that defined MSG, with one of the greater tragedies including the rivalry between Amuro and Char turning into bitter hatred.

https://www.youtube.com/watch?v=lCBWan2FWGU

Annihilation of the Zentraedi fleet (Macross/Robotech) -
this series was the original and still one of the best at how it used juxtaposition to portray its imagery - in this instance, Minmay singing a pop song ("We Will Win") while a massive battle to decide the fate of the Earth is raging. It was a symbol of human civilization, both militarily and culturally, dominating the genocidal alien species. Many anime have tried to mimic this scene, but few - if any - ever truly succeeded.

https://www.youtube.com/watch?v=Hei2efEYLow

-Goku's first Super Saiyan transformation (Dragonball Z) -
this is a no brainer. The moment we saw Goku tremble in grief and rage, then explode into the legendary golden warrior, was awe inspiring. The sheer intensity of the voice actor, along with the looming music that followed, will forever live in the annals of Anime history.

https://www.youtube.com/watch?v=Hw3r0EmHq5w

The Bus Scene (My Neighbor Totoro)
Another unforgettable moment. As Satsuki and Mei are waiting at a rainy bus stop for their father to arrive, Totoro appears and is getting soaked. Satsuki offers the poor thing an extra umbrella, and Totoro is blissfully happy from their kindness. Although many Miyazaki/Ghibli films have iconic moments, this one is the most mimicked and celebrated.

https://www.youtube.com/watch?v=ferxAdJQu8U

"Bang" (Cowboy Bebop)
Another unforgettable moment that probably needs no introduction - but I'll try anyways :P. Gun toting mercenary Spike Spiegel makes his last stand against his former crime syndicate and comrade in arms, Vicious. According to him, not merely for revenge, nor seeking his own death, but "to see if he's still alive." And he ends it all with a.... oh you know it.

https://www.youtube.com/watch?v=lYbYoaBrXpU

One Piece.....
Ok, I'm gonna need some help with this one. One Piece had so many epic moments, I really don't know which one is truly iconic and defines the series. Here's a list of arguably the most memorable OP moments off the top of my head (spoiler alert)